Various chemical, optical, or electronic detectors have been proposed for sensing the presence of gases, liquids, and other analytes. Optical detectors, for example, have been provided that employ a detection layer which is made from a material that modulates or otherwise alters transmitted or reflected light in the presence of an analyte. Reflective layers have been placed adjacent to the detection layer to guide light into or out of the detection layer. When an analyte is present, the detection layer undergoes a responsive change in an optical property. Reflective layers may also provide an indication that the analyte is present (e.g., via optical interference). For example, a calorimetric change may be provided in the presence of a vapor of interest by using a thin-film multilayer indicator having a porous detection layer whose optical thickness changes in the presence of the vapor, located between reflective and semireflective layers at least one of which is permeable to the vapor of interest. One such indicator employs a porous detection layer located between a reflective layer and a vapor-permeable semireflective layer and is described in U.S. Patent Application Publication Nos. US 2004/0062682 A1 (Rakow et al. '682) and US 2004/0184948 A1 (Rakow et al. '948).